The overall goal of this project is to utilize high complexity RNA libraries to isolate RNA-based inhibitors that target cell cycle regulatory proteins. The Rb pathway, which has now been shown to be of critical importance for the control of cell growth, ultimately leading to the regulation of E2F transcriptional activity, will be the focus of this work. Further importance of the Rb pathway in cell growth control is seen by the fact that virtually all human cancers involve alterations in one or another step in the pathway. We and others have employed RNA selection methods to isolate RNA ligands, or aptamers, that can bind to and inhibit the function of diverse proteins. This includes the isolation of RNAs that bind specifically to the E2F transcription factor family, inhibiting DNA binding and resulting in an inhibition of cell cycle progression. Here we propose to employ this RNA selection strategy to isolate RNAs that recognize each of the E2F proteins as well as additional RNAs that target defined gene products that operate in the Rb pathway. We suggest that these RNAs, when synthesized in a modified form that imparts significant stability or when produced from viral vectors, can be valuable reagents for dissecting the functional aspects of this important cell cycle pathway. Moreover, these reagents also have the potential to function as anti-proliferative agents and to provide anti-tumor activity on their own. For the latter case, we propose to develop methods to allow the efficient expression of such RNA ligands through the use of adenovirus vectors. In so doing, we aim to establish the methods necessary to quickly and efficiently isolate RNA ligands that target E2F and other Rb pathway activities to then allow the testing of these molecules as inhibitors of tumor cell growth.